APOD: Solstice Sun and Milky Way (2017 Dec 21)

Comments and questions about the APOD on the main view screen.
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APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by APOD Robot » Thu Dec 21, 2017 5:05 am

Image Solstice Sun and Milky Way

Explanation: Welcome to December's solstice, first day of winter in the north and summer for the southern hemisphere. Astronomical markers of the seasons, solstice and equinox dates are based on the Sun's place in its annual journey along the ecliptic, through planet Earth's sky. At this solstice, the Sun reaches its maximum southern declination of -23.5 degrees today at 16:28 UTC, while its right ascension coordinate on the celestial sphere is 18 hours. That puts the Sun in the constellation Sagittarius in a direction near the center of our Milky Way galaxy. In fact, if you could see today's Solstice Sun against faint background stars and nebulae (that's really hard to do, especially in the daytime ...) your view might look something like this composited panorama. To make it, images of our fair galaxy were taken under dark Namibian night skies, then stitched together in a panoramic view. From a snapshot made on December 21, 2015, the Sun was digitally overlayed as a brilliant star at today's northern winter solstice position, close to the center of the Milky Way.

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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by Ann » Thu Dec 21, 2017 7:32 am

Oh no, the Sun is blotting out the Lagoon and the Trifid!

Great picture! It's fun to see the southerly position of the Sun as seen from the Earth during winter solstice. It would be interesting to see the Sun superimposed on the sky during summer solstice as well.

The colors in this APOD are lovely! I approve of the almost-white Sun.

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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by alter-ego » Thu Dec 21, 2017 7:36 am

For fun I simulated this APOD in Stellarium. I've added our celestial equator (blue), the ecliptic (red) and the galactic equator (tan, horizontal). I forgot about the proximity of the sun to the galactic equator (remember the Mayan Calendar?). Only until the 2019 Winter Solstice will the sun intersect the galactic equator. In the 2020 solstice, its limb will just miss by 2 arcseconds.
2017 Winter Solstice & Sun Location - Stellarium.JPG
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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by ta152h0 » Thu Dec 21, 2017 8:05 am

nicely done
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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by Boomer12k » Thu Dec 21, 2017 9:06 am

Great Image... I hope all our APOD friends are well and happy, and having a great Holiday Season!!!

Merry Solstice...
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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by JohnD » Thu Dec 21, 2017 9:32 am

Is it true that if the stars could be seen in daytime, the Sun would be centered over the Milky Way on the Winter Solstice?
A happy coincidence if so, and a nice picture, but seems most unlikely. More likely that the Sun would be in a random position on the Galactic disc as we see it.
Is there any component of orbital mechanics that would synchronise the Sun, Earth and Galactic Centre all being on a straight line with the Solstice?
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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by nafpie » Thu Dec 21, 2017 11:08 am

Hi John.
JohnD wrote:Is it true that if the stars could be seen in daytime, the Sun would be centered over the Milky Way on the Winter Solstice?
Yes.
JohnD wrote:A happy coincidence ...
Indeed. A 'happy accident'. :wink:
JohnD wrote:Is there any component of orbital mechanics that would synchronise the Sun, Earth and Galactic Centre all being on a straight line with the Solstice?
No. It's by accident.

And it is not forever. The position of winter solistice is moving and returns to the same point every 25,800 years because of the axial precession.

See:
https://en.wikipedia.org/wiki/Axial_precession

Happy solstice!
Stefan

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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by JohnD » Thu Dec 21, 2017 11:31 am

Oooeeeeeeeeer! Weirder and weirder.
The Moon is JUST the right size to cover the Sun at eclipse, and at Winter Solstice we look past the Sun straight at Galactic Centre!
Apart from the charge on the electron and the strength of the weak force being 'just right', what other "coincidences" for our times are there in astronomy?

Not that I know, but a little puzzle for Xmas!
John

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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by nafpie » Thu Dec 21, 2017 11:59 am

JohnD wrote:The Moon is JUST the right size to cover the Sun at eclipse...
Not always. There are annular eclipses too. :mrgreen:
JohnD wrote:Apart from the charge on the electron and the strength of the weak force being 'just right' ...
Good points.

More: Life is very, very unlikely. But it happens after all. Again and again.
Last edited by nafpie on Thu Dec 21, 2017 12:49 pm, edited 1 time in total.

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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by hamilton1 » Thu Dec 21, 2017 12:27 pm

Lovely, lovely image.

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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by ta152h0 » Thu Dec 21, 2017 4:29 pm

that faint little dot must be New Horizons
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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by Chris Peterson » Thu Dec 21, 2017 5:28 pm

nafpie wrote:More: Life is very, very unlikely. But it happens after all. Again and again.
How do you know? (Applies to your first and third assertions.)
Chris

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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by nafpie » Thu Dec 21, 2017 9:28 pm

Chris Peterson wrote:How do you know?
Relax. I didn't speak about 'creation of life'. :wink:

In my mind I had the reproductive biology. The more you learn about, the more unlikely it seems.

But let's talk about the winter solstice instead...

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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by Chris Peterson » Thu Dec 21, 2017 9:36 pm

nafpie wrote:
Chris Peterson wrote:How do you know?
Relax. I didn't speak about 'creation of life'. :wink:

In my mind I had the reproductive biology. The more you learn about, the more unlikely it seems.
I wasn't unrelaxed. Just wondered what you meant. Got to say, though, the complexity of evolved life doesn't seem to me an argument either for or against life in genera.
But let's talk about the winter solstice instead...
We celebrate today as our main winter holiday. Presents under the Solstice Tree this morning, a dinner party tonight.
Chris

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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by neufer » Thu Dec 21, 2017 10:59 pm

Click to play embedded YouTube video.
Chris Peterson wrote:
nafpie wrote:
But let's talk about the winter solstice instead...
We celebrate today as our main winter holiday. Presents under the Solstice Tree this morning, a dinner party tonight.
Oooohh! And another piece of the puzzle falls into place.
Art Neuendorffer

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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by MarkBour » Thu Dec 21, 2017 11:36 pm

alter-ego wrote:For fun I simulated this APOD in Stellarium. I've added our celestial equator (blue), the ecliptic (red) and the galactic equator (tan, horizontal). I forgot about the proximity of the sun to the galactic equator (remember the Mayan Calendar?). Only until the 2019 Winter Solstice will the sun intersect the galactic equator. In the 2020 solstice, its limb will just miss by 2 arcseconds.

2017 Winter Solstice & Sun Location - Stellarium.JPG
Very nice, alter-ego. And I can see in your version (just barely) that Saturn is practically in line with the Sun at the same time.
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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by nafpie » Thu Dec 21, 2017 11:40 pm

Hi Chris.
Chris Peterson wrote:the complexity of evolved life doesn't seem to me an argument either for or against life in genera.
I totally agree.
Chris Peterson wrote:We celebrate today as our main winter holiday. Presents under the Solstice Tree this morning, a dinner party tonight.
Very cool.

:clap:

Stefan

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Re: APOD: Solstice Sun and Milky Way (2017 Dec 21)

Post by nafpie » Thu Dec 21, 2017 11:55 pm

Mark.
MarkBour wrote:I can see in your version (just barely) that Saturn is practically in line with the Sun at the same time.
True!

Today (! Dec 21) Saturn reached the conjunction position (Earth-Sun-Saturn in line). The angular distance of the Sun and Saturn in the sky was only little more than one half degree. Of course, Saturn was far, far behind the Sun.

In fact, it was the greatest distance between Earth and Saturn in this century! 1.652 billion (US) / milliards (GB) kilometer.

Stefan